1. Field of the Invention
The present invention relates to the stabilization of the natural and synthetic rubbers against the deleterious effects of ozone, and to stabilized elastomeric compositions comprising such rubbers.
2. Description of the Prior Art
It is well known to this art that certain polymers, and more particularly the vulcanizates of incompletely saturated elastomers, are highly sensitive to the action of ozone. Among such elastomers can be mentioned the natural and synthetic rubbers, such as the homopolymers and copolymers of conjugated dienes, either with each other or with vinyl comonomers, and particularly polybutadiene, polyisoprene, isobutylene/isoprene copolymers, styrene/butadiene or acrylonitrile/butadiene copolymers, and polychloroprene.
When the rubbery elastomers are subjected to the action of a stress, the damaging effects of ozone are manifested in the appearance of surface cracks oriented at right angles to the direction of the stress. If this stress continues, or each time it occurs, the cracks deepen and can cause total rupture of the vulcanizates. The ozone sensitivity of the subject elastomers is of course a function of various parameters, such as the nature of the elastomer, the ozone concentration in the environment, the temperature and the time of exposure, and the magnitude of the stress to which the rubber is subjected. Atmospheric air contains a significant amount of ozone which can vary according to the geographical locations in question and most articles of vulcanized elastomers are typically subjected to stresses; this is the case, in particular, with automotive tires. The advantage of protecting these vulcanized elastomers to avoid their being damaged too rapidly is thus readily apparent.
Various processes for protecting rubbers against the deleterious effects of ozone have been proposed to this art. Thus, it has been proposed to incorporate into rubbers microcrystalline waxes whose exudation permitted a protective screen to be formed on the surface of the elastomers. This physical barrier is effective only insofar as it is unbroken by external forces, with the result that this mode of protection, excellent for the elastomers employed under static conditions, becomes inoperative when the vulcanized elastomers are subjected to a force under dynamic conditions. In such cases it is preferable to rely on the addition of chemical antiozonant compounds which retard the appearance of cracks under the static and/or dynamic conditions of use of the elastomers.
Very many chemical compounds have been proposed for use as antiozonant agents, but, nevertheless, the problem of protecting rubber vulcanizates has not been solved in a satisfactory manner, with the result that serious need exists in industry for effective antiozonants which do not concomitantly give rise to yet other difficulties. It has been found, in effect, that it is not enough that a chemical compound present a good degree of protection against ozone to constitute a satisfactory stabilizer, if its use is accompanied by disadvantages or drawbacks related to undesirable secondary effects, either at the stage of preparation of the vulcanizates or at the final product stage, or in respect of its handling and its incorporation in the masterbatches. Thus, it too is well known that many materials which offer excellent antiozonant activity are also noted for their tendency to color or stain the vulcanizates in which they are incorporated. Such antiozonants cannot, therefore, be employed for protecting light-colored or white elastomers, or even for protecting black elastomers which are in contact with items of a light color whatever their nature (metallic, rubbery or plastic-based). In the latter case, in fact, the antiozonants are apt to migrate from the elastomer onto the adjoining light-colored parts, on which they generate colorations and/or stains. It has further been determined that many antiozonants disturb the course of the vulcanization, either because they reduce its rate or, to the contrary, because they accelerate it to such a degree that they give rise to the well-known phenomenon of scorching.